1. Localized PEM mRNA and Protein Are Involved in Cleavage-Plane Orientation and Unequal Cell Divisions in Ascidians 
Takefumi Negishi, Tatsuki Takada, Narudo Kawai, Hiroki Nishida 
Current Biology 
Volume 17, Issue 12, Pages (June 2007)
DOI: /j.cub
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2. Figure 1
Cleavage Pattern of Embryos and Morphology of Resulting Larvae Injected with Hr-PEM MO and mRNA
(A–D) Diagrams of embryos at the 4-, 8-, 16-, and 24-cell stages. Anterior is to the left. Orientation of each embryo is indicated. The 24-cell stage is attained after the vegetal blastomeres have divided and before the animal blastomeres divide. The smaller cells formed at the posterior pole by unequal cleavage are shown in pink. The localization of PEM mRNA is shown in red. The gray colored blastomeres in (D) are those of the animal hemisphere. The names of the relevant blastomeres are indicated. Bars connect two sister blastomeres.
(E–I) Control 5-mismatch MO-injected embryos at each stage shown at the top. (E) shows the four-cell stage, and (F) shows the eight-cell stage. The animal pole is up. The B4.1 blastomere protrudes posteriorly. The polar body is indicated by an arrow. (G) and (H) show the sixteen- and twenty-four-cell stages. The smaller B5.2 and B6.3 blastomeres were formed at the posterior pole. (I) shows the normal tail bud developed. The scale bar represents 100 μm.
(J–N) Hr-PEM-MO-injected embryos at each stage. Unequal cleavages did not occur. B5.2 divided vertically into B6.3 and B6.4 blastomeres of equal size. The posterior cleavage pattern is similar to that of the anterior. (N) shows an aberrant tail-bud embryo.
(O–S) Hr-PEM-mRNA-injected embryos at each stage. Cleavage pattern was normal but malformed larvae developed (S).
(T) Sequence of events during the 16-cell stage in the posteriormost blastomere (B5.2) pair that will unequally cleave in the next cell division, corresponding to the illustrations in (C) and (D). Vegetal views are shown. Times after the onset of the previous cleavage are indicated below the illustration. Nuclei are shown in light blue, microtubule in green, and the CAB in red, in which PEM mRNA is localized. See text for details. Modified from [15].
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3. Figure 2
Position of the CAB in Extracted Embryos and Formation of Microtubule Bundles in the 16-Cell-Stage Embryo
(A–C) Control 5-mismatch MO-injected embryos observed at each stage shown at the top. (A) and (A′) show eight-cell-stage embryos, lateral view. CAB is present in the posterior-vegetal B4.1 blastomeres (arrow). (A′) shows a closer view of (A). (B) shows the 16-cell stage, vegetal view. The CAB is located at the posterior pole. (C) shows the 24-cell stage, vegetal view. The CAB is present at the posterior pole in the B6.3 blastomere.
(D–F) Hr-PEM-MO-injected embryos. (D) and (D′) show the eight-cell stage. The CAB is located more vegetally. (E) shows the 16-cell stage, vegetal view. (E′) shows the lateral view; anterior is to the left and the animal pole is up. The CAB is clearly located more vegetally. (F) shows the 24-cell stage. The embryo has the CAB within the blastomere that is not the posterior-most one; instead, the CAB was present within its sister cells closer to the vegetal pole.
(G–L) Formation of microtubule bundles in the 16-cell-stage embryo. Double immunostaining of microtubules (green) and CAB (red) with tubulin and ZF-1 antibodies, respectively. The vegetal view is shown. Anterior is to the left.
(G–I) Control 5-mismatch MO-injected embryo. Arrowheads indicate thick microtubule bundles between a centrosome and the CAB. (I) and (I′) show a merged image and closer view of the posterior region.
(J–L) Embryo injected with Hr-PEM MO into one blastomere at the two-cell stage. (L) and (L′) show that there is no microtubule bundle on right side (arrows).
The scale bars represent the following: in (D′), 50 μm; in (F), 100 μm; in (L), 100 μm; and in (L′), 20 μm. See also Movie S1 for Z stack images of multiple confocal sections of another specimen.
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4. Figure 3
Mitotic Apparatus at M Phase and Astral Microtubules at Interphase during the Four-Cell Stage
(A–E) Immunostaining with tubulin antibody. Anterior is to the left in all figures.
(A, C, and E) Polar (animal or vegetal) views observed with a conventional fluorescence microscope. White lines demarcate shape of embryos. The red dot indicates the position of the animal centrosome, and the red open circle shows that of the vegetal centrosome. (A) shows control 5-mismatch MO-injected embryo. (C) shows embryos injected with Hr-PEM MO into one blastomere at the two-cell stage. One of the posterior spindles is relatively vertical and does not tilt (white arrow). (E) shows an embryo injected with Hr-PEM MO into fertilized egg. Both of the posterior spindles do not tilt.
(A1, A2 and B1, B2) Lateral optical section of each mitotic apparatus observed by confocal microscopy in a control MO-injected embryo. (A1) and (A2) show M phase and are front and back optical sections that correspond to levels 1 and 2, respectively, are indicated in (A). Posterior spindles on both the left and right sides are oblique. (B1) and (B2) show interphase. Inclination of the centrosome axis is already evident.
(C1, C2 and D1, D2) Embryos injected with Hr-PEM MO into one blastomere at the two-cell stage. Posterior spindle and centrosome axis are relatively vertical on one side in (C2) and (D2) (white arrows). Scale bars represent 100 μm.
(F) A model of the role of Hr-PEM in orientation of the third cleavage. One of the centrosomes in the posterior blastomere (with green spindle) is attracted by the posterior-vegetal cortex where Hr-PEM mRNA is present (blue line). As a result, an oblique spindle (green) forms, and eventually the B4.1 blastomeres protrude posteriorly at the eight-cell stage in a normal embryo (upper). Broken lines indicate the planes of the next division. Rotation of the centrosome axis does not occur in Hr-PEM knockdown embryos (lower). The two photos at right show cleaving embryos during the third division, in which the cleavage plane of the posterior blastomere (red arrowheads) is obliquely oriented in control embryo, but not in PEM-MO-injected embryos.
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5. Figure 4
Positions of ZF-1 and Hr-PEM mRNA Localization and Positions of the Polar Body
(A–D′) Localization of ZF-1 mRNA in embryos injected with control and Hr-PEM MOs. Stages and orientation of embryos are indicated. Anterior is to the left in vegetal and lateral views. The animal pole is up in posterior views. ZF-1 mRNA (white arrows) localized in the PVC is detectable close to the posterior-pole region within the posterior blastomeres in control embryos. In embryos injected with Hr-PEM MO, the ZF-1 signal is shifted vegetally. Scale bars represent 100 μm.
(E–H′) Localization of Hr-PEM mRNA. Similar vegetal shift of the localization is observed.
(I and J) Position of polar bodies (red arrow) in uninjected eight-cell-stage embryos. θ is the angle between the polar bodies and the second division plane. Polar bodies are preferentially located in the a4.2 blastomere in (J).
(K and L) Embryo injected with Hr-PEM MO. Polar bodies are located evenly in the a4.2 and b4.2 blastomeres in (L).
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6. Figure 5
Model for Control of Orientation and Positioning of the Cell Division Plane by Hr-PEM
(Upper) In a normal embryo at the two-cell stage, the centrosome axis rotates as the posterior centrosome is attracted toward the posterior-vegetal cortex (blue cortex), where Hr-PEM protein is localized (red dots on blue cortex), and a tilted spindle (green) forms. The second division plane is oblique to the A-V axis (large red vertical arrow). Broken lines indicate the planes of the next division. At the four-cell stage, similar events occur again, only in the posterior blastomeres. Therefore, the B4.1 blastomere protrudes posteriorly at the eight-cell stage. Polar bodies are located preferentially in the a4.2 blastomere. The A-V axis passes through the a4.2 and B4.1 blastomeres. The A4.1 and b4.2 blastomeres would inherit both animal (white) and vegetal (yellow) cytoplasm from the egg. In the B4.1 blastomeres, the centrosome/nucleus complex approaches the CAB, and the blastomere divides unequally. At the 16-cell stage, a smaller B5.2 blastomere forms at the posterior pole. This posterior-most blastomere undergoes two further unequal cleavages.
(Lower) Without Hr-PEM protein, the cleavage pattern is quite regular because there is no centrosome-attracting activity. With respect to the cleavage pattern, no remarkable events occur. The second division plane simply forms parallel to the A-V axis, and the third cleavage becomes latitudinal. In the eight-cell embryo the CAB appears to be located more vegetally in the B4.1 blastomeres. In the 16-cell-stage embryo, all blastomeres are of the same size.
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7. Figure 6
Movements of Astral Microtubules during the Second Cleavage in a Living Embryo
(A and B) Bright-field images of fertilized egg and time-lapse images of the same embryos obtained by confocal microscopy during the second cleavage. Times after initiation of recording are shown in each right corner. See also Movies S2 and S3 corresponding to (A) and (B). Rhodamine-conjugated tubulin was injected in order to visualize microtubules. Lateral views are shown. Bright-field images taken at the one-cell stage reveal the position of the polar bodies at the top (white arrows). Thus, the animal pole is up. Posterior is to the right because clear cytoplasm is located in a relatively posterior region of the egg at this stage [12]. (A) shows control without MO injection. The posterior centrosome is attracted toward the posterior-vegetal cortex during the two-cell stage, resulting in rotation of the centrosome axis (red arrows), and a tilted spindle forms. Eventually, the second division plane (green arrowheads) becomes oriented obliquely to the A-V axis. A third light spot is present in the anterior region at 0:15 and 0:28. This spot probably represents fluorescent tubulin unincorporated into asters. The brilliant tiny spot on top of the embryo at 0:15 and 0:28 is not polar body. Polar body is invisible in fluorescence images because we injected fluorescent tubulin after polar-body emission. (B) shows a Hr-PEM-MO-injected embryo. Rotation of the centrosome axis did not occur, and the second cleavage plane was vertical. The scale bar represents 100 μm.
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8. Figure 7 Localization of Hr-PEM Protein and Hr-PEM mRNA
(A–C and A′–C′) Hr-PEM protein was immunohistochemically detected with the specific antibody. The staining pattern was confirmed in more than 15 specimens. Developmental stages and orientations of embryos are indicated. Anterior is to the left. In (A) and (A′), staining is observed in CAB-forming region and the CAB. Arrowheads indicate the CAB-forming region at the two-cell stage. In (B), (B′), (C), and (C′), Hr-PEM protein was detected in extracted embryos. Specific signal persisted after extraction with Triton and glycerol in control MO-injected embryos (B and B′). Unfertilized egg and fertilized egg were extracted and fixed before injection of control MO. Staining in CAB-forming region and the CAB was lost in Hr-PEM-MO-injected embryos (C and C′).
(D and D′) Localization of Hr-PEM mRNA detected by in situ hybridization. Embryos were cleared with benzyl alcohol and benzyl benzoate after in situ hybridization. The scale bar represents 100 μm.
Current Biology  , DOI: ( /j.cub )
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